Microglial cells induce degeneration of photoreceptor cells from transgenic mice

Purpose. In animal models of retinal degeneration, microglial cells have been observed in the subretinal space during photoreceptor cell death. Although several mechanisms have been advanced to explain the loss of photoreceptors, studies in our lab suggest that microglial cells may be involved in this process. Thus, we have begun to investigate the effects of microglial cells on photoreceptor cells in vitro. The difficulty of obtaining pure populations of photoreceptor cells has necessitated the use photoreceptor cells (661w) generated from retinas of transgenic mice expressing the SV40 T antigen directed by hIRBP promoter. Methods. 661w cells were treated with basal medium (DMEM with 0.1% bovine serum albumin) or basal medium conditioned by activated microglial cells (MGCM) or Müller cells (MCCM). The number of live cells and dead cells were assayed following treatment for 24-72 hrs. using non-radioactive colorimetric techniques. Results. About 70% of 661w cells grown in MGCM were dead after 72 hrs. of treatment as compared with 20% of those treated with MCCM or 25% of those grown in basal medium. Supplementation of the media with 1% serum resulted in similar findings. In addition, the number of surviving, cells following MCCM treatment was 1.35x higher as compared with those grown in basal medium. Conclusions. Our study shows that microglial cells induce degeneration of photoreceptor cells from transgenic mice and supports our hypothesis that microglial cells are responsible for the induction of photoreceptor cell death in degenerative retinal diseases. Moreover, our study shows that this event involves the release of soluble cytotoxic products by microglial cells. Interestingly, the proliferation of the 661w photoreceptor cells in Müller cell-spent medium suggests that Müller cells may secrete trophic factors that promote the proliferation and/or survival of photoreceptor cells.